In an automotive air conditioning system having an automatic control device, the control device conventionally includes a control circuit which compares a predetermined air temperature, an automobile compartment air temperature and an ambient air temperature to control the opening angle of an air mixing damper. The air mixing damper is operated by an actuator which regulates the degree of the opening of the damper in accordance with an output signal received from the control circuit to thereby control the temperature of the air blown into the automobile compartment from an air duct of the air conditioning system.
In this type of automotive air conditioning system, an ambient air temperature sensor for detecting ambient air temperature at the exterior of the vehicle is conventionally attached at a portion of the front grill of an automobile so that the ambient air temperature can be accurately detected while the automobile is being driven. However, when the automobile is idling, the ambient air temperature sensor will detect a higher temperature than the actual ambient air temperature because the detected ambient air temperature is affected by the wasted heat of the automobile engine.
To resolve this problem, it has been conventionally proposed to use an ambient air temperature sensor which has a large heat capacity so that the rate at which the sensor responds to temperature changes is delayed, or alternatively to employ a control circuit which delays the response to a temperature change, detected by an ambient air temperature sensor which has a small heat capacity, for a certain time. As a result of delaying the response time to a variance in temperature, the opening angle of the damper can be prevented from being improperly increased.
In spite of the above solution, if the automobile idles for a long time, the ambient air temperature sensor will become sufficiently heated by the wasted heat of the automobile engine, that the ambient air temperature data which is detected by the ambient air temperature sensor will nonetheless become high as compared with the actual ambient air temperature. Then, when the automobile is thereafter driven, the ambient air temperature surrounding the ambient air temperature sensor will quickly decrease to the actual ambient air temperature. However, the ambient air temperature data which is maintained by the control device will not quickly equal the actual ambient air temperature because the conventional control device is predetermined to delay any response for a certain time. Thus, the opening angle of the damper will be smaller than desired until the detected ambient air temperature data equals the actual ambient air temperature, and the heating of the automobile compartment will be insufficient for a certain period of time.